Many elevator systems include an elevator car and counterweight that are suspended within a hoistway by roping comprising one or more load bearing members. Typically, a plurality of ropes, cables or belts are used for supporting the weight of the elevator car and counterweight and for moving the elevator car to desired positions within the hoistway. The load bearing members are typically routed about several sheaves according to a desired roping arrangement. It is desirable to maintain the load bearing members in an expected orientation based upon the roping configuration.
There are other vertically extending members within many elevator systems. Tie down compensation typically relies upon a chain or roping beneath an elevator car and counterweight. Elevator systems typically also include a traveling cable that provides power and signal communication between components associated with the elevator car and a fixed location relative to the hoistway.
There are conditions where one or more of the vertically extending members such as the load bearing member, tie down compensation member or traveling cable may begin to sway within an elevator hoistway. This is most prominent in high rise buildings where an amount of building sway is typically larger compared to shorter buildings and when the frequency of the building sway is an integer multiple of the natural frequency of a vertically extending member within the hoistway. There are known drawbacks associated with sway conditions.
Various proposals have been made for mitigating or minimizing sway of a vertically extending member within a hoistway. One example approach includes using a swing arm as a mechanical device for inhibiting sway of a load bearing member, for example. U.S. Pat. No. 5,947,232 shows such a device. Another device of this type is shown in U.S. Pat. No. 5,103,937.
Another approach has been to associate a follower car with an elevator car. The follower car is effectively suspended beneath the elevator car and is positioned at the midpoint between the elevator car and a bottom of a hoistway for sway mitigation purposes. A significant drawback associated with this approach is that it introduces additional components and expense into an elevator system. In addition to the follower car and its associated components, the size of the elevator pit must be larger than is otherwise required, which takes up additional real estate space or introduces additional costs or complexities in designing and building the elevator shaft. Additionally, follower cars have only been considered to mitigate sway of compensation ropes and they introduce additional potential complications into an elevator system.
Another approach includes controlling the position of an elevator car and the speed with which the car moves within a hoistway for minimizing the sway. It is known how to identify particular elevator car positions within a hoistway corresponding to particular building sway frequencies that will more effectively excite the vertically extending members. One approach includes minimizing the amount of time an elevator car is allowed to remain at such a so-called critical position when conditions conducive to sway are present. Various elevator movement control strategies are described in WO 2007/013434 and WO 2005/047724.
While the previous approaches have proven useful, those skilled in the art are always striving to make improvements.